As the process for the production of 2-thiophene aldehydes, there have hitherto been known various processes; for example, 2-thiophene aldehydes are obtained by the Vilsmeier reaction using formamides and phosphorous oxychloride (see, e.g., U.S. Pat. No. 2,853,493), or formamides and acid chlorides such as phosphorous oxychloride are used under the presence of pyridine (see, e.g., JP-A 56-133220).
In the Vilsmeier reaction using phosphorous oxychloride, however, when thiophene derivatives each having a substituent at the 3rd position thereof are used, a mixture of the 2-formyl product and the 4-formyl product at a ratio of 4:1 is formed with the percent yield of the 2-formyl product being only about 33%; therefore, for the purpose of obtaining 2-thiophene aldehydes, such a mixture of the products should be separated (see, e.g., J. Chem. Soc. Perkin Trans I, 1973, 2327). In addition, these conventional processes have disadvantages in that they lead to the formation of phosphorous-containing waste water of great volume and that it is necessary to recover pyridine which is then to be converted into anhydrous pyridine; therefore, they are not always satisfactory from an industrial point of view.